As explained in the PR description, they existed only to reliably get a pointer to self in a time with managed headers in some owned boxes.

Furthermore, they polluted the global namespace if you use `Any`, as every type suddenly gets a `as_[mut_]void_ptr()` method for something that should only be an implementatin detail.

Finally, their usage meant un-erasing a type from an `Any` trait object needing two virtual calls, instead of just one.